Well, I took out my old stat, and I think I found my problem (looked like this in the engine block):

 

A bit of JB weld and it'll be as good as new.

 

Eh. You could probably go for a few more years on that t-stat...

Find a replacement in the stock temp range for best overall performance. Doesn't have to be the overpriced OEM version. Any good parts store will have a quality replacement unit.

The viscous clutch with the mechanical fan won't free spin like it's on bearings. But it should spin easily when cold. When you fire up the engine, you should be able to tell when it gets to the heat level where the clutch should kick in and the fan speed kicks up a notch. That's the more important part. As long as it engages at higher temps, you can live with it. The only reason to have the viscous clutch is to improve fuel mileage when the engine doesn't need the fan running full blast.